The present invention relates to a method and system to search for a critical path to identify critical paths in various large scale integrated circuits (LSI). In this specification, a xe2x80x9csemiconductor devicexe2x80x9d means a logic circuit, memory circuit and analog circuit, or a whole semiconductor device composed of any combination thereof.
Today, the number of transistors integrated into LSI""s has dramatically increased. Particularly, the circuits of logic LSI""s including microprocessors as a typical device, memories, analog LSI""s and system LSI""s have been becoming increasingly complicated. In order to perform failure analyses on these large scale, complicated and further high speed LSI and to feed-back the results to device design, efforts to search out critical paths in the LSI""s have been frequently made. The critical path means particular paths limiting the operating speed of the whole circuit in a signal propagation path in an LSI. For the design of an LSI, it is important to control the propagation time of these paths below predetermined values.
Hitherto, searches for a critical path in an LSI has been performed by using simulations based on the design data of an LSI. For each of circuits constituting an LSI, the simulation technology can calculate from the design data the propagation time to pass through various arithmetic circuits, memories and others from an input of a signal to the output. Therefore, when a predetermined test vector (a test pattern) is inputted, a calculation can obtain how it operates in the inside of the LSI and thus allows a search for a critical path.
On the other hand, the previous technique to obtain a critical path in an LSI by using simulation technology as described above has the following problems.
(1) It takes too much time to make a program which performs a simulation for finding out the critical paths for all logic signals.
(2) Because it does not operate actual circuits (semiconductor devices), it takes too much time to set a load and others that are impossible or difficult for simulations to represent.
(3) Because it performs the processing of massive amount of numerical data, it takes very much time for a search.
The present invention has been created in view of these problems. The object of the invention is to provide a method and system to search for a critical path which allows a quick and reliable detection of a critical path by using an actual semiconductor device.
A preferred embodiment of the method and system to searching for a critical path in accordance with the present invention is based on the following. The period of operation clock pulses is decreased up to a period T1 on which the device can not operate normally. Then, the operating clock pulse corresponding to the position of a critical path is changed to a little longer period T2, so that the semiconductor device can be normally operated. In consideration of this point, it is examined which pulse of n operating clock pulses between the input of data and the output of the data has been changed from T1 to T2 for normally operating the semiconductor device. Thereby, the search for a critical path is performed.
Specifically, an operating clock producing means sets the periods of the n operating clock pulses to T1 or T2, and an test data input means inputs data to the semiconductor device. The semiconductor device operates corresponding to the inputted data in synchronization with the n operating clock pulses and outputs data. An output data determining means determines whether the output data from the semiconductor device is wrong or correct. When this output data is correct, that is when the semiconductor device normally operates, a search control means decides that a critical path exists at the position of the operating clock pulse with the period T2 and performs search processing for a critical path.
In this way, the present invention performs the search for a critical path by actually operating a semiconductor device. Compared to the case of searching for a critical path by a simulation technique, the search according to the present invention allows fast and reliable detection of a critical path. Further, the search of the present invention can operate a semiconductor device with the setting of a load and others and so allows search processing in consideration of actual operating conditions.
Further, by using the search control means described above, each period of the operation clock pulses from the nth pulse to the (nxe2x88x92i)th pulse and a period of the other are set to T2 and to T1, respectively, and whether the semiconductor device operates normally or not is examined. The value of (nxe2x88x92i) on which the semiconductor device operates normally is identified as a critical path generation start position. This is a first target. Also, by using the search control means, a period of the operation clock pulses included in a predetermined range including the critical path generation start position as the leading position thereof and a period of the other are set to T2 to T1, respectively. Then, whether the semiconductor device operates normally or not is examined. The narrowest range in which the semiconductor device operates normally is identified as a critical path generation segment. This is a second target. In this way, the present invention identifies a critical path generation start position and critical path generation segment and thereby allows an accurate search for a critical path generation position. As a result, it becomes easy to take measures such as a design change.
Yet further, the present invention is suitable for the search for a critical path in a semiconductor device including a PLL circuit which produces another internal clock pulses in synchronization with operating clock pulses inputted from the outside. The search method for critical path in accordance with the present invention also can operate a semiconductor device on a period T1 and T2 of operating clock pulses that are placed in the vicinity of the boundary of whether the semiconductor device operates normally or not. So, when the periods of the operation clock pulses inputted from the outside are changed between T1 and T2, the internal clock pulses produced by the internal PLL circuit are easy to follow the operating clock pulses. Therefore, the search method of the present invention, which searches for a critical path by changing the periods of the operation clock pulses as described above, is readily applicable to such semiconductor device.